undetected virus in avast

[avast_user_01]

I am running Microsoft Windows XP Home Edition, Version 2002, Service Pack 2.  Recently I am continually experiencing a trojan/virus which appears to be undetected by my up to date version of Avast antivirus software.

The virus changes my dial up number to an 0870------- number and various other dial up pre configured settings and  also alters my login username.  This can be corrected by deleting my dial up connection and re-entering the details.  the user is completely unaware of this unless he  views his network connection manually.
Q1. Why is this virus undetected.
Q2. What can be done to correct this.

[Lisandro]

Which is the name and the path of the infected file(s)?
I mean, if it is undetected, are you sure that another security software detects it?
For instance, a-squared, ewido or Spyware Terminator (trojan removers).

Can't you run on-line scanning?
Trendmicro
Virustotal
ewido

[DavidR]

If you haven't already got this software (freeware), download, install, update and run it.
1. Ad-Aware
2. Spybot Search and Destroy
3. Spywareblaster (Blaster provides proactive protection against diallers). Don't install this until you are clean otherwise the dialler that is on your system would effectively avoid this protection.